A synchronous, clocked communication system having characteristics of equidistance is understood to be a system of at least two stations which are connected to one another via a data network for the purpose of the mutual exchange of data or, the mutual transmission of data. In this arrangement, the data exchange is cyclic in equidistant communication cycles which are predetermined by the communication clock used by the system. Stations are, for example: centralized programmable controllers, programming, configuration or operating devices, peripheral devices such as e.g. input/output modules, drives, actuators, sensors, stored-program controllers (SPS) or other control units, computers or machines which exchange electronic data with other machines, particularly those which process data of other machines. In the text which follows, the control units are understood to be closed-loop or open-loop control units of any type. The data networks used are, for example, bus systems such as, e.g. field bus, process field bus, Ethernet, industrial Ethernet, FireWire or also internal PC bus systems (PCI) etc.
In distributed automation systems, for example in the field of drive engineering, clocked data networks or bus systems are often used. Some of the connected stations are used as master devices, e.g. control units such as numeric or stored-program controllers or configuration devices, and other stations are used as slave devices such as drives or peripheral devices. Automation components of both categories can operate in a clocked manner, i.e., these stations can synchronize to a communication clock used in the data network. This means that the communication clock can be taken over by the stations via the data network used and certain processes can be controlled in synchronism with this communication clock. According to IEC 61491, EN61491 SERCOS interface—Technische Kurzbeschreibung (basic technical description) (http://www.sercos.de/deutsch/doku_freier_bereich.htm), this is currently used and carried out in distributed automation systems. In contrast, decentralized input/output modules such as, e.g. I/O modules which, as interfaces, for example between the data network and devices to be controlled or a process to be controlled, bidirectionally ensure the exchange of signals and/or data between the devices or processes to be controlled and other stations of the communication system such as, e.g. control units, by means of the data network, currently cannot operate in a clocked manner, i.e. they cannot synchronize to a communication clock of the communication system. As a result, it is not possible to switch inputs and outputs of the decentralized input/output modules at a fixed predetermined time and, on the other hand, it is also not possible to specify the switching of outputs with sufficiently great timing accuracy. As a result, the events of signal detection at a device or process to be controlled and data output to the device or a process to be controlled cannot be deterministically linked to one another and the time interval between signal detection and data output depends on the dead times within the communication system, especially within the decentralized input/output modules. In the text which follows, the term device to be controlled also means a process to be controlled, especially an automation process and/or subprocess of an automation process.